Sensor for in-situ pad wear during CMP

ABSTRACT

Wear of a CMP polishing pad is detected in-situ by monitoring a change in the height of an identification mark positioned on a rotatable spindle supporting a wafer in contact with the chemical mechanical polishing pad. As the pad experiences wear, its thickness decreases and the relative height of the identification mark declines. This change in height can be detected by optical interrogation of the identification mark, and then calibrated to reveal a precise amount of pad wear. Alternatively, changes in thickness of the polishing pad can be correlated to a drop in dielectric properties of the pad as monitored by an electrical sensor measuring capacitance. Apparatuses for sensing pad wear in accordance with the present invention can also be utilized to evaluate the quality of new polishing pads, to indicate the optimum time for replacing worn polishing pads, and to identify the point at which a worn polishing pad begins to damage a wafer being polished.

BACKGROUND OF THE INVENTION

[0001] The present invention generally relates to Chemical Mechanical Polishing (CMP). In particular, it relates to apparatuses and methods enabling in-situ measurement of pad wear during CMP processes.

[0002] Polishing of semiconductor wafers by CMP during fabrication of integrated circuits is an accepted practice in the semiconductor industry. Typically, a wafer to be polished is secured to a head supported by a spindle, and then placed into contact with a polishing pad in combination with an abrasive slurry. Over time, continued frictional contact between the wafer and the polishing pad alters the character of the surface of the polishing pad. This in turn affects the quality of the wafer polishing resulting from the frictional contact.

[0003] The surface of the polishing pad is typically regenerated through periodic contact with a separate conditioning disk containing diamond abrasive. The resulting fresh surface of the polishing pad produced by contact with the conditioning pad is important to ensure the effectiveness and consistency of polishing.

[0004] The material forming the polishing pad is relatively soft and may be abraded or worn away through repeated contact with the conditioning disk and the wafer. Conventionally, the issue of wear of polishing pads has been addressed by replacing the polishing pads at regular intervals, for example after approximately every 1000 wafers polished.

[0005] However, this conventional periodic replacement approach is based upon only a generalized estimate of pad wear. In reality, pad wear is dependent upon a host of variables, including the quality of the pad, wafer-to-wafer variation in operational parameters of a CMP process, and tool-to-tool variation in operational parameters of a CMP process. Over-frequent replacement of CMP polishing pads wastes pad material and increases the consumables cost of the CMP process. Conversely, under-frequent replacement of CMP polishing pads may result in uneven polishing and the loss of wafers in production.

[0006] Therefore, methods and structures permitting direct measurement of the wear of polishing pads in a CMP device are desirable.

SUMMARY OF THE INVENTION

[0007] Embodiments of the present invention provide structures and methods permitting in-situ measurement of wear of CMP polishing pads. Structures and methods for detecting pad wear may utilize sensors that monitor physical changes in pad thickness. One approach utilizes an optical sensor detecting a change in the relative height of a mark positioned on a rotatable spindle supporting the wafer in contact with the polishing pad. As the pad experiences wear and exhibits reduced thickness, the relative height of the spindle mark will drop. This change in height can be calibrated to a given amount of pad wear. In an alternative embodiment in accordance with the present invention, changes in polishing pad thickness can be correlated to a drop in dielectric properties of the pad as detected by an electrical sensor. Apparatuses and methods for sensing wear in CMP polishing pads according to embodiments of the present invention can also be utilized to evaluate the quality of new polishing pads, or to indicate the optimum time for replacement of polishing pads.

[0008] An embodiment of a method for detecting wear of a chemical mechanical polishing pad in accordance with the present invention comprises detecting in-situ a change in thickness of a chemical mechanical polishing pad by sensing a change in a position of a spindle supporting the wafer in contact with the chemical mechanical polishing pad. This change in pad thickness is then correlated with an amount of pad wear.

[0009] An alternative embodiment of a method for detecting wear of a chemical mechanical polishing pad in accordance with the present invention comprises detecting in-situ a change in thickness of a chemical mechanical polishing pad by sensing an increase in capacitance resulting from application of a potential difference across the worn polishing pad. This change in pad thickness is then correlated with an amount of pad wear.

[0010] An embodiment of a method of determining the quality of a chemical mechanical polishing pad in-situ in accordance with the present invention comprises applying a compression force to spindle supporting the wafer in contact with the polishing pad, detecting a distance of displacement of the spindle toward the polishing pad, and correlating the distance of movement with compression distances of prior polishing pads of known quality.

[0011] An embodiment of an apparatus for detecting in-situ wear of a CMP polishing pad comprises a spindle supporting a head and a wafer in contact with a chemical mechanical polishing pad. A visual identification mark positioned on the spindle. A light source is configured to illuminate the visual identification mark. A detector is configured to receive light reflected from the visual identification mark, such that a reduction in thickness of the pad causes a change in position of the visual identification mark that may be correlated with an amount of pad wear.

[0012] An alternative embodiment of an apparatus for detecting wear of a chemical mechanical polishing pad in-situ comprises a platen supporting a chemical mechanical polishing pad in contact with a wafer. A power source has a first terminal in electrical communication with the platen and the second terminal in electrical communication with the wafer. A detector is configured to sense a change in capacitance between the first and second terminals, such that a reduction in thickness of the pad causes a change in capacitance that may be correlated with an amount of pad wear.

[0013] These and other embodiments of the present invention, as well as its features and some potential advantages are described in more detail in conjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 shows a cross-sectional view of a chemical mechanical polishing apparatus.

[0015]FIG. 2 shows a cross-sectional view of an apparatus for detecting pad wear in accordance with one embodiment of the present invention.

[0016]FIG. 3 plots polyurethane pad wear versus time.

[0017]FIG. 4 plots compression distance versus applied compression force for normal and defective CMP pads.

[0018]FIG. 5 shows a cross-sectional view of an apparatus for detecting pad wear in accordance with an alternative embodiment of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0019]FIG. 1 is a side view of a chemical mechanical polishing apparatus. CMP apparatus 100 includes chemical mechanical polishing pad 104 of thickness Y supported by platen 102. Platen 102 and polishing pad 104 are in contact with and rotatable about axle 106. Chemical mechanical polishing slurry 108 is dispensed from vessel 110 to the surface of pad 104.

[0020] Rotatable carousel 118 supports first spindle 116 and second spindle 120. Head 114 supports wafer 112 in contact with pad 104, and is secured to and rotatable about first spindle 116. Pad conditioning disk 124 is secured to and rotatable about second spindle 120. Conditioning disk 124 typically features a diamond abrasive useful in regenerating chemical mechanical polishing pad 104 after use.

[0021] Specifically, contact between polishing pad 104 and conditioning disk 124 roughens the surface of polishing pad 104 from a worn state following a prior polishing operation, and readies polishing pad 104 for a subsequent polishing operation. A 180° rotation of carousel 118 enables continuous operation of CMP device 100, by allowing polishing pad 104 to be exposed to conditioning disk 124 while second polishing pad 130 is in turn utilized to polish the next wafer secured to head 114.

[0022]FIG. 2 shows a simplified side view of an apparatus for detecting pad wear in accordance with one embodiment of the present invention. Wear detection apparatus 200 includes head 202 supporting wafer 206 that is to be polished. Polishing pad 204 having an initial thickness Y is supported by platen 203. Rotatable spindle 207 biases head 202 and wafer 206 into contact with chemical mechanical polishing pad 204 with a loading force F of between approximately 2-4 psi.

[0023] Spindle 207 bears identification mark 208 located at a fixed position. Initially, identification mark 208 is positioned at height h.

[0024] Wear detection apparatus 200 further includes optical interrogation device 210 (such as a laser or other light source) that is directed at visual identification mark 208. Initially, mark 208 lies at height h and light from source 210 is reflected back to optical detector 212. However, as polishing pad 204 experiences wear, it is reduced in thickness. Accordingly, spindle 207 drops and the height of identification mark 208 is reduced. Because the path of light from optical interrogation device 210 remains fixed, light from device 210 is no longer fully reflected by mark 208. The decrease in intensity of reflected light sensed at detector 212 can be calibrated at controller 213 to a drop in the height of the mark and hence a change in thickness of the polishing pad attributable to pad wear. In making this pad wear determination, controller 213 can take into account the change in mark position attributable to reduced thickness of the wafer due to wafer polishing.

[0025]FIG. 3 plots pad wear versus time of a polyurethane polishing pad tested against a diamond conditioning disk of a CMP apparatus. The pad was maintained in contact with the conditioning disk at a constant pressure of approximately 0.5 psi. Voltage levels on the y-axis indicate the intensity of light received by the optical detector.

[0026]FIG. 3 reveals a steady decline in pad thickness resulting from continuous abrasion of the polishing pad by the conditioning disk. FIG. 3 illustrates that wear of a CMP polishing pad can be constantly monitored in-situ. This quantification of pad wear enables determination of the optimum time for pad replacement for each individual pad. This analysis further enables monitoring of the quality of pads received from vendors, as poor quality pads will wear at an excessive rate.

[0027] The apparatus of FIG. 2 may allow polishing pad quality to be determined even before the pad is exposed to polishing conditions. Specifically, the compressibility of a polishing pad—its shape change in response to an applied force—may reveal the quality of the pad material. Compressibility of a CMP polishing pad may be measured by detecting the change in height of a visual identification mark on a spindle in response to a loading force of known magnitude applied to the axle to bias the head and wafer against the polishing pad.

[0028] Accordingly, FIG. 4, plots displacement distance D (in μm) versus applied compression force F (in Newtons), for normal and defective CMP pads. FIG. 4 shows that a defective pad exhibits a substantially larger displacement distance in response to the same applied compression force. Again, displacement distance can be measured through optical interrogation of an identification mark positioned on an axle supporting a polishing pad.

[0029] While the invention has been described so far in connection with determination of pad wear through an optical sensor directed to an axle supporting the pad, the invention is not limited to this particular embodiment. Other sensor types could be employed to measure pad wear in-situ, and the resulting method or apparatus would fall within the scope of the present invention.

[0030] For example, FIG. 5 shows an alternative embodiment in accordance with the present invention, wherein thickness Y of polishing pad 504 may be determined by applying a potential difference across pad 504 and measuring its resulting dielectric property.

[0031] Specifically, one terminal of power supply 502 is connected to head 505 supporting wafer 506, and the other terminal of power supply 502 is connected to platen 508 supporting polishing pad 504. As a result of this configuration, capacitor structure 512 is created having as plates wafer 506 and platen 508, and having as a dielectric chemical mechanical polishing pad 504. As pad 504 experiences wear, its thickness decreases. The distance Y separating wafer 506 and platen 508 correspondingly decreases. The correlation between pad thickness and capacitance can be calibrated to reveal an amount of pad wear.

[0032] Only certain embodiments of the present invention are shown and described in the instant disclosure. One should understand that the present invention is capable of use in various other combinations and environments and is capable of changes and modification within the scope of the inventive concept expressed herein. For example, while the invention has been discussed above in connection with identifying pad wear for identifying pad replacement and the level of pad quality, one should understand that the present invention could be employed for other purposes, such as identifying the point at which a worn pad may begin to damage the wafer being polished.

[0033] Given the above detailed description of the present invention and the variety of embodiments described therein, these equivalents and alternatives along with the understood obvious changes and modifications are intended to be included within the scope of the present invention. 

What is claimed is:
 1. A method for detecting wear of a chemical mechanical polishing pad comprising: detecting in-situ a change in thickness of a chemical mechanical polishing pad by sensing a change in position of a spindle supporting a head and a wafer into contact with the chemical mechanical polishing pad; and correlating the change in pad thickness with an amount of pad wear.
 2. The method of claim 1 wherein the change in position is detected by monitoring a change in an intensity of light reflected from a visual identification mark positioned on the spindle.
 3. The method of claim 1 further comprising correlating the amount of pad wear with a rate of pad wear of polishing pads of known quality in order to identify a quality of the polishing pad.
 4. A method for detecting wear of a chemical mechanical polishing pad comprising: detecting in-situ a change in thickness of a chemical mechanical polishing pad by sensing an increase in capacitance resulting from application of a potential difference across the worn polishing pad; and correlating the change in pad thickness with an amount of pad wear.
 5. The method of claim 4 further comprising correlating the amount of pad wear with a rate of pad wear of polishing pads of known quality in order to identify a quality of the polishing pad.
 6. A method of determining in situ a quality of a chemical mechanical polishing pad material, the method comprising: applying a compression force to a spindle supporting a wafer into contact with the polishing pad; detecting a distance of displacement of the spindle toward the polishing pad in response to the compression force; and correlating the displacement distance with displacement distances of prior polishing pads of known quality.
 7. The method of claim 6 wherein the displacement distance is sensed from a change in a height of a visual identification mark located on the spindle.
 8. The method of claim 7 wherein the displacement distance is sensed from a change in intensity of light from a light source that is reflected from the visual identification mark.
 9. The method of claim 6 wherein the displacement distance is detected from an increase in a capacitance resulting from application of a potential difference across the polishing pad.
 10. An apparatus for detecting pad wear in-situ comprising: a platen supporting a chemical mechanical polishing pad; a head supporting a wafer into contact with the chemical mechanical polishing pad; a spindle in contact with the head; a visual identification mark positioned on the spindle; a light source configured to illuminate the visual identification mark; and a detector configured to receive light reflected from the visual identification mark, such that a reduction in thickness of the pad causes a change in position of the visual identification mark that may be correlated with an amount of pad wear.
 11. An apparatus for detecting wear of a chemical mechanical polishing pad in-situ comprising: a platen supporting a chemical mechanical polishing pad in contact with a wafer; a power supply having a first terminal in electrical communication with the platen and the second terminal in electrical communication with the wafer; and a detector configured to sense a change in capacitance between the first and second terminals, such that a reduction in thickness of the pad causes a change in capacitance that may be correlated with an amount of pad wear. 